For setting the throttle flap angle of an internal combustion engine, electronic control devices are used which vary the throttle flap angle as a function of the position of the accelerator pedal and of parameters specific to the engine, such as speed, temperature and the like. In addition, the idle stop, which fixes the idling throttle flap angle, is varied by means of a positioning motor as a function of the engine temperature and of further parameters. In this way, the idling speed is influenced or controlled. However, in the event of a malfunction, the throttle flap may be opened up to relatively large angles, resulting in considerable operational disturbances. The event of a malfunction may occur, for example, in the case of a short-circuit of a motor triggering line of the positioning motor to battery voltage or ground.
In order to be able to establish such malfunctions, the state of the equipment output terminals which serve to trigger the positioning motor of the idle stop could be monitored by means of appropriate circuit arrangements. A deviation from the desired state could be detected by a computer and corresponding counter measures could be initiated. The disadvantage of such a monitoring device is the additional circuitry and the occupation of inputs of the control, which could then no longer be used for other purposes.
U.S. patent application Ser. No. 410,586, filed on Sept. 21, 1989 discloses a method for controlling the idle speed of an engine via the position of the throttle flap. The angle of the throttle flap is changed with a movable idle stop of the flap in dependence upon operating parameters such as temperature, engine speed and alike with this change being such that the idle speed takes on a pregiven value determined from the operating parameters.
On the other hand, U.S. Pat. No. 4,622,936 discloses an electronic gas pedal in association with an electronic fuel controller for an internal combustion engine. The position of the throttle flap is determined in dependence upon the position of the accelerator pedal via a control arrangement and a positioning motor. For this purpose, a desired value for the throttle flap position is determined from the accelerator pedal and is compared with an actual value generated by a position transducer and corresponding to the actual position of the throttle flap. The position of the throttle flap is then changed in dependence upon this difference.
U.S. Pat. No. 4,452,200 discloses an idle speed control system which operates by adjusting the position of the throttle flap. A position desired value is formed in dependence upon operating parameters and is compared to a value determined from the position of the throttle flap and a motor is driven for changing the position of the throttle flap with the aid of the signal representing this difference. For detecting the non-idle speed condition, the throttle flap position value is stored in memory when the sign of the error signal changes into negative. The stored value is then compared with an actual position value and the non-idle speed condition can be detected if the difference between the stored and actual throttle flap value exceeds a predetermined threshold value.
Finally, U.S. Pat. No. 4,580,220 discloses a fail-safe emergency operation device for an idle speed control. In this device, the drive pulses of the positioning component influencing the idle speed is fed back to the control unit generating the drive signal. A comparison determines whether an error condition is present in the area wherein the drive signal is formed.